1. Field of the Invention
The present invention relates to a display device including matrix wiring and, more particularly, to an active matrix display device for preventing static electricity.
2. Description of the Related Art
Flat-panel display devices, especially liquid crystal display devices, have come to be used in various fields because they are thin, light, and of low power consumption. Among those display devices, active matrix liquid crystal display devices in which switching elements are provided for respective display pixels are used in fields where high-resolution display images are particularly required, because crosstalk between adjacent pixels can be minimized.
For example, an active matrix liquid crystal display device is constructed in such a manner that a twisted nematic (TN) liquid crystal component is held between an array substrate and a counter substrate via alignment films.
In the array substrate, a plurality of signal lines and a plurality of scanning lines are arranged in matrix form with an insulating film interposed therebetween, and active elements such as thin-film transistors (TFTs) serving as switching elements are arranged in the vicinity of the respective crossing points of the signal lines and the scanning lines. Pixel electrodes are connected to the lines via the active elements. The counter substrate has a counter electrode that confronts the pixel electrodes.
Usually, an insulating substrate made of glass, quartz, or the like is used as the array substrate. However, because the substrate is made of insulator, the insulating substrate is influenced by static electricity that is generated during manufacture. That is, static electricity causes a high voltage to be applied to crossing points of signal lines and scanning lines, possibly causing an insulation failure. This results in such problems as short-circuiting between a signal line and a scanning line and a device failure of a thin-film transistor.
To solve the above problems, a technique is known which prevents local application of a high voltage by distributing charge that is accumulated on the signal lines and the scanning lines by connecting those lines to a common conductor wiring (short ring). However, since the short ring needs to be removed at the final manufacturing step, this technique cannot reduce the rate of occurrence of manufacturing failures due to static electricity that occur after the final step.
In view of the above, as disclosed in, for instance, JP-A-63220289 (Japanese Unexamined Patent Publication No. Sho 63-220289), another technique has been developed in which rather than the signal lines and the scanning lines are connected to the short ring directly, they are connected to the short ring via switching elements. For example, the switching elements establish an electrical path between the signal lines/scanning lines and the short ring only when a voltage difference that is larger than a voltage difference corresponding to ordinary operation voltages is generated. This technique can greatly decrease manufacturing failures due to static electricity because it is not necessary to remove the short ring.
However, concentrated studies of the present inventors revealed that the above technique increases current consumption.
Incidentally, a liquid crystal display device has been proposed which can reduce voltage drops in the signal lines and which enables the output impedance of an external driving circuit for applying voltages to the signal lines to be set at a higher value than in conventional cases (JP-A-09090428 (Japanese Unexamined Patent Publication No. Hei. 9-90428)).
In this liquid crystal display device, electrostatic breakdown protective elements are connected between the scanning lines and a protective element line and between the signal lines and a protective element line. Further, the protective element lines are connected to a common line to which a common voltage is applied via resistor elements. The resistor element is formed by making the line width of a portion of a wiring line smaller than that of the other portions by etching and also making the film thickness smaller there. With this configuration, currents flowing into the scanning lines from the common line via the electrostatic breakdown protective elements are reduced by the resistor elements, whereby the common voltage can be prevented from decreasing.
However, even this type of liquid crystal display device has the following problems.
First, since wiring line portions serve as the resistor elements, they are prone to influences of line disconnection or the like.
Second, to provide a sufficiently large resistance, it is necessary to make wiring line portions sufficiently thin and long. Since the formation of such wiring line portions are subject to exposure-related limitations and are influenced by various kinds of variations, it is difficult to form resistor elements in a stable manner. Further, long wiring line portions are subject to limitations on the positions of their formation.
An object of the present invention is therefore to provide an active matrix display device which can prevent the production yield from decreasing due to the influence of static electricity as well as can lower the power consumption.